At the present time, voice communication such as telephone speech communication is popular through the use of various networks. Nevertheless, frequencies of telephone speech communication are typically limited to frequencies from 300 Hz to 3.4 kHz, generally called “telephone band”, according to customs in the days of conventional public networks. However, the human voice contains low-band components of 300 Hz or lower and high-band components of 3.4 kHz or higher. These low-band components and high-band components are important components which may influence voice personality. Moreover, for the elderly people, lack of these low-band components and high-band components not only causes lack of voice personality but also makes speech understanding difficult. Thus, it is desired that the telephone speech communication covers these components.
However, a telephone exchange system in a general public network cannot transmit an audio signal outside the telephone band. In view of this point, Patent Document 1 proposes a band extension device for extending an audio frequency band.
The method of the band extension device described in Patent Document 1 will be described with reference to FIG. 2. A narrow-band audio signal (digital signal) DC whose frequency is limited to a range from 300 Hz to 3.4 kHz is inputted to the band extension device 10. The narrow-band audio signal DC is converted to a converted original signal S whose sampling frequency (e.g., from 8 kHz to 16 kHz) is raised by the sampling frequency converter 11. Through the use of the converted original signal S, an extended signal (a low-band signal for combination) LS extended to a lower band side (300 Hz or lower), an extended signal (a high-band signal for combination) HS extended to a higher band side (3.4 kHz to 7 kHz), and an extended signal (a soundless-section signal for combination) US obtained by extending a soundless section are generated by a low-band signal generator 12, a high-band signal generator 13, and a soundless-section signal generator 14, respectively. These signals are added to the above-mentioned converted original signal S by the adder 15 to generate a band-extended signal V.
The band-extended signal V provides a transmitted signal concurrently with a low-band component signal, a high-band component signal or the like, which are generated from a band-limited narrow-band audio signal DC. Therefore, one can hear a sound with more realism that is similar to a sound generated from a wide-band signal containing these low-band and high-band component signals.
Patent Document 1 is Japanese Patent Application Kokai (Laid Open) Publication No. 9-258787.